The present invention relates generally to switching regulators. More particularly, the invention relates to a new topology for a positive buck-boost switching regulator.
Buck-boost switching regulators are a common solution to the problem of providing a regulated output voltage from a widely varying input voltage. They are frequently implemented in battery powered electronic devices to regulate the battery output voltage which, during charging and discharging, can be greater than, less than, or substantially the same as the desired output voltage.
Conventional buck-boost switching regulators generally have one or more power switches and an inductor-capacitor (LC) filter coupled between an unregulated input and a regulated output via power switches. A control circuit selects the buck-boost switching regulator configuration (i.e., the operating mode) by controlling switch positions of the power switches and the duration of time during which switch positions remain unchanged.
Depending on the switch configurations, the buck-boost switching regulator can operate as one of three distinct converter types: a buck converter, a boost converter, or a buck-boost converter. A buck converter down-converts an input voltage to a lower output voltage. The power switches operating in a buck converter configuration alternately connect and disconnect the input voltage from the input of the LC filter. A boost converter, on the other hand, up-converts an input voltage to a higher output voltage. In the boost converter configuration, the input supply is continuously connected to the input of the LC filter, but the inductor of the LC filter is alternately connected and disconnected from the load where the regulated output voltage is applied. A buck-boost converter switches between the buck-converter configuration and the boost converter configuration.
A typical control circuit operates the buck-boost switching regulator as a buck converter, boost converter, or buck-boost converter depending on whether the input voltage is above, below, or substantially the same as the desired output voltage. Feedback is introduced to allow the control circuit to monitor the output voltage and adjust the switching regulator configuration accordingly. The control circuit operates to not only control the configuration the buck-boost switching regulator is operating in, but the duration of time the configuration remains unchanged. Controlling these two parameters allows the buck-boost switching regulator to regulate the output voltage at a desired reference voltage level.
A pulse-width modulated (PWM) control scheme is typically used to manage the time duration (i.e., duty cycle) the buck-boost switching regulator configuration remains unchanged, as well as the actual configuration the regulator operates in: buck, boost, or buck-boost. An error signal, proportional to the difference in output voltage and the desired reference voltage level, is used by the PWM controller in these determinations.
A disadvantage of this typical buck-boost switching regulator topology is that the configuration (i.e., the operating mode) of the switching regulator is determined within the PWM control circuit. The PWM control circuit typically contains a compensation network for stabilizing the feedback loop introduced when calculating the error signal. Switching from buck to boost is therefore dependent on the compensation network. As a result, the transient response of the buck-boost switching regulator is slow.
A further disadvantage of this typical buck-boost switching regulator topology is that the selection of the PWM controller is restricted by the necessity of the buck-boost switching regulator configuration (i.e., the operating mode) being determined within the controller.
Furthermore, typical buck-boost switching regulators maintain an average inductor current that is significantly larger than the required output current when the input voltage is close to the regulated output voltage. As a result, conduction losses are high and efficiency is decreased.
It would therefore be desirable to provide a high efficiency buck-boost switching regulator topology that separates the operating mode circuitry from the PWM control circuitry. It would also be desirable to provide a circuit for determining the operating mode of the buck-boost switching regulator that is not dependent on any feedback signals from the switching regulator circuit.
It would be further desirable to provide a buck-boost switching regulator topology that maintains the inductor current as close as possible to the output current. This reduces conduction losses and increases efficiency when the input voltage is close to the regulated output voltage.